Space elevator trips could be agonisingly slow

The simple act of climbing could throw space elevators off track and potentially into harm’s way, a new study suggests. Fixing the problem could require agonisingly slow trips lasting nearly a month or the careful choreography of multiple climbers.

Space elevators have been proposed as a cheap alternative to expensive rocket launches to send cargo, and perhaps even humans, into space.

The elevators would be made of a cable (also called a tether or ribbon) that would be anchored to the Earth’s surface and balanced by a counter-weight in space. Lasers on Earth would beam power to “climbers” that would crawl up the tether with their cargo.

But the concept has been stuck on the ground floor for decades, not least because current materials are not strong enough to handle the strain on the tether. Carbon nanotubes may work, but they need to be longer and purer than those manufactured today.

Even with adequate materials, space elevators might be highly unstable. Gravitational tugs from the Moon and Sun, as well as pressure from gusts of solar wind, could shake the tether, potentially causing the elevator to crash into nearby satellites or space junk. Thrusters might be needed to keep the tether in line.

Now, it seems that the act of climbing itself could also cause the tether to wobble.

Earth’s spin

The culprit is the Coriolis force, which deflects objects that are moving in a rotating system. On Earth, the planet’s rotation sets up a Coriolis force that deflects air currents and water.

In this study, the space elevator would stand vertically from a point on Earth’s equator. As the climber ascended, its motion would cause the Coriolis force to pull the climber, and thus the cable, in the opposite direction of Earth’s rotation.

This would pull the elevator away from its vertical resting position, causing it to oscillate back and forth like a pendulum, say mechanical engineers Arun Misra of McGill University in Montreal, Canada, and Stephen Cohen, now at MDA Space.

Wrong orbits

The motion of cargo up the elevator might only cause the cable to wobble back and forth by a fraction of a degree, says Misra.

But the tether’s swing could either boost or reduce the velocity of any spacecraft exiting the elevator. That could send them into orbits that are tens of kilometres too high or too low, Misra says, adding that significant amounts of fuel might be needed to correct the problem.

“Because it’s a huge structure, this small deviation can make large errors in the orbit that will be reached by the payload,” Misra told New Scientist.

The wobbles induced by climbing cargo might also help send the elevator into harm’s way.

Slow crawl

The faster a climber goes, the larger the effect, Misra told New Scientist, so the simplest way to minimise a climber’s effect on the cable is to slow its ascent.

Slowing the climb would allow the space elevator to be drawn back to its stable point, perfectly vertical above the Earth’s equator. But this could also make trips into orbit agonisingly slow, adding nine days or more to a climb that – at several hundred kilometres per hour – might already take about 15 days.

Alternatively, the speed of climbs might be fine-tuned as the trip progresses, Misra says. Multiple climbers could also be sent up the tether at the same time, in a carefully orchestrated ballet.

“I think we are finding that building a space elevator is a lot more complicated than simply stringing up a ribbon and whizzing up and down it,” says Anders Jorgensen of the New Mexico Institute of Mining and Technology in Socorro, US, who has studied space elevator stability.

Speed limit

“Needing to impose a particular climber schedule and speed could have implications for the economical viability of a space elevator, so it is a very important question to answer,” Jorgensen says.

“Traffic-phasing will likely play a role,” agrees David Lang, a consultant based in Seattle, Washington, who has also studied the effects of climbers.

But other methods, involving coupled space elevators arranged side by side or one-way traffic that occasionally reverses direction, might also help. “I would say that it is too early to establish the best way to do this,” Lang told New Scientist.